The Messenger of Allah ﷺ said, "When a servant commits a sin, a black spot is marked upon his heart. If he refrains, seeks forgiveness, and repents, his heart is polished clean. But if he returns to sin, the spot grows until it overtakes his heart. That is the rust which Allah mentioned, 'Nay, but on their hearts is the rust of what they used to earn' [Mutaffifin 83:14]."He said, this is a hasan sahih hadith.
حدثنا قتيبة حدثنا الليث عن ابن عجلان عن القعقاع بن حكيم عن أبي صالح
عن رسول الله ﷺ قال إن العبد إذا أخطأ خطيئة نكتت في قلبه نكتة سوداء فإذا هو نزع واستغفر وتاب سقل قلبه وإن عاد زيد فيها حتى تعلو قلبه وهو الران الذي ذكر الله {كلا بل ران على قلوبهم ما كانوا يكسبون} قال هذا حديث حسن صحيح
No references are available.
Takhrij, hasan. Nasai narrated it in Sunan al-Kubra, no. 11658, from Qutayba, and Ibn Majah, no. 4244, from the hadith of Ibn Ajlan through this route. Busiri graded it sahih, as did Ibn Hibban, no. 1771, 2448, and Hakim, according to the criterion of Muslim, 2/517, and Dhahabi agreed with him. Ibn Ajlan narrated it using the ambiguous form "an," and the hadith has corroborating narrations.